New York City has been placed on high alert after a bomb ripped through a busy street in the Chelsea neighborhood on Saturday night.

Mayor Bill De Blasio and New York police commissioner James O'Neill said at a press conference on Sunday afternoon that there would be an added police presence around the city after the bombing - but stopped short of calling it terrorism.

'It was an intentional, violent, criminal act,' De Blasio said, adding that it was 'organized'.

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Mayor Bill De Blasio and New York police commissioner James O'Neill said at a press conference on Sunday afternoon that there would be an added police presence around the city after the bombing

New York City has been placed on high alert after a bomb ripped through a busy street in the Chelsea neighborhood on Saturday night. Pictured is the scene of the explosion on Sunday

After the initial blast at 8.30pm, an undetonated device was found two hours later by state troopers on West 27th Street.

The NYPD said that a controlled explosion of this device was taking place on Sunday between 4.30pm and 6.30pm near Rodman's Neck in the Bronx.

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O'Neill said that more officers would be circulating on the city's transit system, more police dogs would be used in patrols and random bag searches were being conducted.

Gordon and his three friends were taken to Mount Sinai West, on 58th Street and 10th Avenue, around 10.30pm. Pictured, firefighters arrive at the scene of the explosion

Gordon said he didn't notice anything suspicious and it was just 'a normal night walking through the streets'. Pictured, police officers look for suspicious packages on Fifth Avenue

The National Guard were out in force at Penn Station on Sunday and heavily-armed members of the NYPD's Hercules team, a Special Forces-like counter-terrorism unit, were seen at the entrance to subway stations.

At one event going on in the city on Sunday, Little Italy's Feast of San Gennaro celebration, police officers manned crash barriers set up around trash cans. A mobile command unit and a number of police vans were also on the scene.

The site of the explosion, on 23rd Street, remained closed to the public late on Sunday afternoon as investigators collect hundreds of pieces of evidence.